Lecture 6 Flashcards
What are the names given to the two regions of the embryo lower order animals (not humans) that give rise to the three germ layers and extraembryonic structures
Epiblast – gives rise to the germ layers, hypoblast – forms vegetal pole/hemisphere
In which regions of the morula would you find the structures that give rise to the embryo and to the extraembryonic structures
Top of the morula gives rise to the epiblast, bottoms gives rise to the hypoblast
What changes govern the formation of these two layers in the morula
Governed by morphogens and transcription factors that lead to changes in gene transcription
What causes the hollowing of the morula during development alongside squashing
Changes in osmolarity
Which region of the morula ultimately ends up undergoing apoptosis
Hypoblast
At which stage does the primitive streak in chick embryos form at the posterior of the embryo
Stage 3
At stage 4 of chick development, the primitive streak has elongated and Hensen’s node has formed anteriorly, what does this structure then begin to express
Expresses and secretes BMP antagonists such as chordin, noggin and follastatin
At what stage in chick development does the notochord begin to form
Stage 5-6
What structure then extends forward from the node and is involved in anterior-posterior axis formation
The notochord
Why are the events from morulation to gastrulation less studied in mammalian embryos and why is this
Mammalian embryos have been less extensively studied because they are difficult to maintain in culture after the blastula stage due to this being the stage at which they normally implant into the uterus
At five days post-fertilisation what structure forms in the mammalian embryo that is similar to the hypoblast
Trophoblast
What day corresponds to chick development stage 2 and corresponds to the embryo implantation into the uterus lining
12 days post-fertilisation
At what stage in mammalian embryonic development is the embryo referred to as a blastula
12 day stage
The processes occurring in chick embryos from stage 2-6 occur in 2-4 week old human embryos, T or F
T
At which region of the developing embryo does gastrulation and germ layer formation begin at
Posterior of the embryo
What is the first structure to become visible in the initial stages of gastrulation
Formation of the primitive streak due to cells moving into the midline forming a line that elongates anteriorly
What forms at the most anterior point of the primitive streak
(Hensen’s – in chicks) node
Which regions of the embryo does the primitive streak denote
Posterior/caudal regions
The epiblast gives rise to mesoderm, endoderm and ectoderm, T or F
T
Explain how signals from the hypoblast layer of the developing embryo accounts for the formation of three germ layers by the overlying epiblast
Signals from the hypoblast (trophoblast) induce some epiblast cells to become mesoderm and definitive endoderm. These cells involute and ingress along the anterior-posterior axis by losing contact with one another and migrating into the embryo. Superficial migrated cells become the mesoderm and the first cells to have ingressed, those now lying deeper within the epiblast will become endoderm.
How does the migration of cells into the embryo at the primitive streak account for the formation of both mesoderm and endoderm
Cells that migrated first from the epiblast layer will lie deeper within the embryo and will become the endoderm. Those that migrated later will be lying for superficially and will become mesoderm.
Once primitive streak reaches maximum extension and forms Hensen’s node, a similar event takes place whereby cells ingress but begin to move directly anteriorly/rostrally, T or F
T
What is different about the gastrulation of Xenopus embryos compared with mammal/human early embryonic development
The Xenopus embryo doesn’t compact to form a blastodisc as mammalian embryos do. It remains morula-like with an animal cap and a vegetal hemisphere
What structures in the Xenopus embryo correspond to the epiblast and hypoblast seen in mammalian embryos
Animal cap – epiblast, vegetal hemisphere – hypo/trophoblast
Explain how oocytes are already polarised before fertilisation
Particular determinants inside the egg have become localised in the vegetal hemisphere of the cell. This is due to gravity in Xenopus and an interaction with the placenta in mammals
What is the result of the inherent polarisation of the egg prior to fertilisation
Different cytoplasmic determinants have sunk to one part of the egg. This leads to the cells that come from this region being different and having specific factors localised/activated
In an early cleavage event, the vegetal and animal hemispheres of the now fertilised egg are separated, what is the effect of this on cytoplasmic determinant localisation
Cytoplasmic factors restricted to the vegetal part of the oocyte are now restricted to vegetal hemisphere cells
What is the effect of the factors that become localised to the vegetal hemisphere cells of the embryo as a result of early cleavages in the embryo that separate the animal and vegetal cells
These factors localised in the vegetal part of the embryo bind to promoters for particular transcription factors and upregulation. In-turn, these particular transcription factors become solely expressed in vegetal hemisphere cells
What is the name of the transcription factor that is specifically expressed and localised to vegetal hemisphere cells
VgT
Describe what happens as a result of the localisation of a specific transcription factor in the nuclei of cells in the vegetal hemisphere of the early embryo
Localised VgT in the nuclei of cells in the vegetal hemisphere leads to its binding to the promoter of a gene called Nodal and upregulate its expression. The Nodal morphogen then leaves the cells of the vegetal hemisphere and binds to receptors on cells in the animal hemisphere
What is the name of the morphogen expressed in the vegetal hemisphere cells that is upregulated as a result of VgT binding to its promoter
Nodal
What is the effect of the morphogen produced by cells of the vegetal hemisphere
Nodal binds to receptor on cells in the animal hemisphere and dictates the mesoderm and endoderm formation via cell proliferation, migration and differentiation.
What happens to cells at the top of the animal hemisphere as a result of VgT activity on its promoter
The Nodal morphogen produced as a result of this leaves the cells and binds to receptors in the animal hemisphere. It forms a concentration gradient throughout the animal hemisphere. Cells at the top of the animal hemisphere don’t receive nodal signalling and so remain as ectoderm. This is thought to be due to either not expressing the nodal receptors or by not receiving a threshold concentration of the nodal ligand
Nodal is expressed throughout the vegetal hemisphere/hypoblast in the developing embryo. What aspects/axes of early embryonic development does this signalling mechanism not account for
Can’t account for anterior-posterior axis formation and the appearance of the primitive streak at the distal region of the embryo
Which signalling pathway is therefore responsible for anterior-posterior patterning and formation of the primitive streak
Wnt signalling
